The present invention relates to a hydraulic control system for a continuously variable V-belt transmission.
A conventional continuously variable V-belt transmission is described in Japanese provisional publication No. 52-98861 which corresponds to GB-PS No. 1,525,674. In this known continuously variable V-belt transmission, the reduction ratio is controlled by a shift control valve which is actuated by a drive pulley revolution speed signal fluid pressure and an engine output signal fluid pressure. The drive pulley speed signal fluid pressure is generated by a pitot tube disposed in fluid within a flume rotatable with the drive pulley. In the case of this continuously variable V-belt transmission, the drive pulley is directly connected to an engine (i.e., a change-direction gearing is not disposed between the engine and the drive pulley) so as to rotate in the same direction as the engine does. Therefore, what is necessary for the pitot tube is to detect the drive pulley revolution speed only when the drive pulley rotates in the forward direction. However, in this case, since change-direction gearing is disposed downstream of the pulleys of the continuously variable V-belt transmission mechanism and thus is subject to a multiplied torque which has been increased by the pulleys, it is necessary for the change-direction gearing to have a large torque capacity. As compared to change-direction gearing in a continuously variable V-belt transmission where the change-direction gearing is disposed upstream of the pulleys, the torque capacity of the change-direction gearing disposed downstream of the pulleys must be n times greater, where n is the largest reduction ratio of the continuously variable V-belt transmission. This causes a problem in that the change-direction gearing must be bulky.
A good solution to the above mentioned problem is to arrange the change-direction gearing upstream of the pulleys of the continuously variable V-belt transmission mechanism. In this case, however, when a conventional pitot tube is used to generate a fluid pressure signal indicative of revolution speed, the pulley revolution speed signal fluid pressure is not generated by the pitot tube when the pulley rotates in the reverse despite the fact the pulley rotates not only in the forward direction but also in the reverse direction. When the pulley rotates in the reverse direction, the pulley revolution speed signal fluid pressure is zero and the reduction ratio between the pulleys of the continuously variable V-belt transmission mechanism cannot be shifted. Thus, it has been the conventional practice to set the reduction ratio of the continuously variable V-belt transmission, for example, the largest (maximum) reduction ratio when the drive pulley rotates in the reverse direction, so that if the continuously variable V-belt transmission is applied to a vehicle, a problem arises in that a large noise is made and fuel economy is poor during reverse running of the vehicle.